Image intensifier tubes are used in night vision devices to amplify light and allow a user to see images in very dark conditions. Night vision devices typically include a lens to focus light onto the light receiving end of an image intensifier tube and an eyepiece at the other end to view the enhanced imaged produced by the image intensifier tube.
Modern image intensifier tubes use photocathodes. Photocathodes emit electrons in response to photons impinging on the photocathodes. The electrons are produced in a pattern that replicates the original scene. The electrons from the photocathode are accelerated towards a microchannel plate. A microchannel plate is typically manufactured from lead glass and has a multitude of microchannels, each one operable to produce a cascade of secondary electrons in response to an incident electron.
Therefore, photons impinge on the photocathode producing electrons which are then accelerated to a microchannel plate where a cascade of secondary electrons are produced. These electrons impinge on a phosphorous screen, producing an image of the scene.
A drawback to this approach is that the electrostatic fields in the image intensifier are not only effective in accelerating electrons from the photocathode to the microchannel plate and from the microchannel plate to the screen, but also move any positive ions back to the photocathode at an accelerated velocity. Current image intensifiers have a high indigenous population of positive ions. These are primarily due to gas ions in the tube, including in the microchannel plate and the screen. These include both positive ions and chemically active neutral atoms. When these ions strike the photocathode, they can cause both physical and chemical damage. This leads to short operating lives for image intensifiers.
To overcome this problem, an ion barrier film can be placed on the input side of the microchannel plate. This ion barrier is able to block the ions from the photocathode. One drawback of the ion barrier is that it reduces the signal-to-noise ratio of the image intensifier. This is due to the fact that the barrier prevents low energy electrons from reaching the microchannel plates.
Therefore, current image intensifiers require an ion barrier since current manufacturing techniques fail to remove enough gas molecules. But the presence of the ion barriers reduces the signal-to-noise ratio. What is needed is an unfilmed microchannel plate that has a sufficient number of gas ions removed such that an image intensifier manufactured with such a microchannel plate has a usable life.
Modern image intensifier tubes also provide automatic brightness control (ABC) and bright source protection (BSP). ABC maintains a relatively constant level of brightness in the image produced by the image intensifier tube despite fluctuating levels of brightness in the scene being viewed. BSP prevents the image intensifier tube from being damaged by high levels of current that may otherwise be generated in response to an extremely bright source.
Conventional image intensifier tubes provide ABC and BSP by adjusting the voltage level of the microchannel plate. Thus, for a brighter scene, the voltage to the microchannel plate is reduced. A drawback to this approach is that the image intensifier tube loses resolution as the voltage to the microchannel plate is reduced. What is needed is an image intensifier tube that provides ABC and BSP without a loss in resolution as the brightness increases.